1. Field of the Invention
The present invention relates to a device for driving a piezoelectric element.
2. Description of the Related Art
A well known device for charging and discharging a piezoelectric element is adapted to charge an electric charge to a power condenser of a piezoelectric element through a charging coil, and discharge the electric charge in the piezoelectric element through a discharging coil, but this device has a problem such that the discharged electric charge is wastefully consumed.
To solve the above problem, a device for charging and discharging a piezoelectric element is known in which a part of the discharged electric charge is recovered by a power condenser, to reduce the power consumption (refer to Japanese utility model publication No. 62-117251).
The above device for charging and discharging a piezoelectric element, in principle, comprises a first discharging circuit for discharging an electric charge in a piezoelectric element PZT to the ground through a discharging coil L2, and a second discharging circuit for discharging an electric charge in the piezoelectric element PZT to a high potential side of a power condenser C, through the discharging coil L2, as shown in FIG. 10. The power condenser C is charged by a high voltage generator E, and when a switching means S1 is turned ON, the electric charge in the power condenser C is charged to the piezoelectric element PZT through a charging coil L1. During a discharging operation, a switching means S3 is initially turned ON and the electric charge in the piezoelectric element PZT is discharged through the second discharging circuit, so that the discharged electric charge is recovered by the power condenser C. Then, the switching means S3 is turned OFF and a switching means S2 is turned ON, so that the electric charge in the piezoelectric element PZT is discharged through the first discharging circuit. Accordingly, since the power condenser C is thus charged not only by the high voltage generator E but also by the discharged electric charge, the power consumption can be reduced.
When the terminal voltage of the piezoelectric element PZT during the discharging operation becomes a high negative voltage, the polarization state of the piezoelectric element PZT is changed, the amount of expansion of the piezoelectric element is reduced, and the piezoelectric element is heated whereby the polarization becomes poor. Therefore, it is necessary to control the terminal voltage of the piezoelectric element PZT during the discharging operation so that a high negative voltage does not occur. Further, since the amount of expansion of the piezoelectric element PZT can be controlled by the terminal voltage of the piezoelectric element PZT during the discharging operation, preferably the terminal voltage of the piezoelectric element PZT is freely set during the discharging operation. Nevertheless, although the device for charging and discharging a piezoelectric element as disclosed in the above Japanese Unexamined Utility Model Publication No. 62-117251 can to a certain extent prevent the terminal voltage of the piezoelectric element PZT from becoming a negative voltage during a discharging operation, it cannot completely prevent this phenomenon. Namely this device has less control of the terminal voltage of the piezoelectric element, during the discharging operation, to thus obtain a predetermined positive voltage. This problem will be further discussed with reference to FIG. 10 and FIGS. 11(A) to 11(F).
In FIG. 10, the electrostatic capacity of the power condenser C is much larger than that of the piezoelectric element PZT, and the high voltage generator E generates a voltage as high as 320 (V). The terminal voltage of a high potential side of the power condenser C is, therefore, substantially 320 (V), and when the switching means S1 is turned ON in this state, the piezoelectric element PZT is charged and the terminal voltage of the piezoelectric element PZT becomes as high as 600 (V). In such a condition, the switching element S3 is initially turned ON, and then the switching element S2 is turned ON, and thereafter, a discharging operation is started. The change of the terminal voltage of the piezoelectric element PZT at this time is shown in FIGS. 11(A), 11(B) and 11(C).
FIG. 11(A) shows a state wherein the switching element S3 is turned ON for a short time. When the switching element S3 is turned on, the electric charge in the piezoelectric element PZT is recovered by the power condenser C through the second discharging circuit, but since the switching means S3 is turned ON for only a short time, there is little drop in the terminal voltage V of the piezoelectric element PZT during this time. Then, when the switching means S3 is turned OFF and the switching means S2 is turned ON, the electric charge in the piezoelectric element PZT is discharged through the first discharging circuit, and since the piezoelectric element PZT and the discharging coil L2 constitute a resonance circuit at this time, the terminal voltage V of the piezoelectric element PZT after the discharging operation is completed becomes a negative voltage. The magnitude of the negative voltage is almost 1/3 of the terminal voltage V (positive voltage) of the piezoelectric element PZT when the switching means S2 is turned ON and the discharging operation is started in the first discharging circuit. Namely, as exemplified in FIG. 11(A), since the terminal voltage V of the piezoelectric element PZT when the switching means S2 is turned ON is slightly lower than 600 (V), the terminal voltage of the piezoelectric element PZT after the discharging operation is completed becomes substantially -200 (V).
FIG. 11(B) shows an example wherein the switching means S3 is turned ON for a longer time, i.e., where the switching means S3 is turned OFF and the switching means S2 is turned ON when the terminal voltage V of the piezoelectric element PZT drops to as low as 400 (V). In this case, the negative voltage generated in the terminal of the piezoelectric element PZT after the discharging operation is completed is slightly lower, at approximately -150 (V).
FIG. 11(C) shows an example wherein the switching means S3 is turned ON until the discharging operation through the second discharging circuit is completed, and thereafter, the switching means S2 is turned ON. When the switching means S3 is turned ON, the piezoelectric element PZT, the power condenser C and the discharging coil L2 constitute a resonance circuit, and at this time, the terminal voltage of the piezoelectric element PZT after the discharging operation is completed is also lower by about 1/3 of the terminal voltage V of the piezoelectric element PZT when the discharging operation is started, where the terminal voltage (approximately 320 (V)) of a high potential side of the power condenser C is taken as a reference, than the terminal voltage of a high potential side of the power condenser C. Namely, since the terminal voltage V of the piezoelectric element PZT where the terminal voltage of a high potential side of the power condenser C when the discharging operation is started is regarded as a reference is approximately 300 (V), the terminal voltage V of the piezoelectric element PZT after the discharging operation is completed becomes approximately 200 (V), and as long as the switching means S2 is not turned ON, the terminal voltage V of the piezoelectric element PZT is maintained at approximately 200 (V). FIG. 11(C) shows an example wherein the switching means S2 is turned ON when the terminal voltage V of the piezoelectric element falls to approximately 200 (V). In this case also, the negative voltage generated in the terminal of the piezoelectric element PZT after the discharging operation is finished becomes approximately 1/3 of the terminal voltage V of the piezoelectric element PZT at the time of the start of the discharging operation, and therefore, the negative voltage generated at the terminal of the piezoelectric element PZT after the completion of the discharging operation is as low as -80 V. In this case, if the switching means S2 is turned OFF before the discharging operation is completed, i.e., if the ON time of the switching means S2 is short, energy stored in the discharging coil L2 is abruptly discharged when the switching means S2 is turned OFF, and thus the switching means S2 is damaged. Namely, the switching means S2 cannot be, thus turned OFF until the discharging operation is completed, and thus the terminal voltage V of the piezoelectric element PZT after the discharging operation is completed becomes approximately -80 (V). Accordingly, if the electric charge charged in the piezoelectric element PZT is discharged through the second discharging circuit, and then discharged through the first discharging circuit, the terminal voltage V of piezoelectric element PZT after the completion of the discharging operation cannot be maintained at a negative voltage lower than approximately -80 (V), and further, cannot be maintained at a constant positive voltage.
Nevertheless, if, after the electric charge in the piezoelectric element PZT is discharged through the first discharging circuit it is discharged through the second discharging circuit, the terminal voltage V of the piezoelectric element PZT can be set at any voltage between -200 (V) and +200 (V). This will be now explained with reference to FIGS. 11D, 11E and 11F. Namely, when the switching means S2 is turned ON, the discharge of the electric charge in the piezoelectric element PZT is started through the first discharging circuit, and therefore, the terminal voltage V of the piezoelectric element PZT begins to drop. Then, when the switching means S2 is turned OFF and the switching means S3 is turned ON, the discharging operation is started through the second discharging circuit.
As shown in FIG. 11(D), when the ON time of the switching means S2 is short, the result is equal to that obtained when only the switching means S3 is turned ON. Accordingly, at this time, the terminal voltage V of the piezoelectric element PZT after the completion of the discharging operation becomes as high as 200 (V), as explained with reference to FIG. 11(C).
On the other hand, when the ON time of the switching means S2 is made longer, the result is equal to that obtained from the example shown in FIG. 11(A), and at this time, the terminal voltage V of the piezoelectric element PZT after the completion of the discharging operation becomes approximately -200 (V).
Next, an example wherein the ON time of the switching means S3 is between the short and long times will be described. This case is more easily explained by using energy, as an example, and thus the following will be based on energy.
When the switching means S2 is turned ON and the discharge of the electric charge in the piezoelectric element PZT is started, the piezoelectric element PZT continues to emit energy and the terminal voltage V of the piezoelectric element PZT continues to fall. When the piezoelectric element PZT begins to emit energy, a current is to flow caused continuously through the discharging coil L2 in the direction shown by arrows indicating the first discharging circuit, and this energy is gradually accumulated in the discharging coil L2. Then, when the switching means S2 is turned OFF and the switching means S3 is turned ON, the energy accumulated in the discharging coil L2 is applied to the power condenser C via the second discharging circuit, and thus the power condenser C is charged. Since the current is caused to flow continuously through the discharging coil L2 for sometime after the switching means S2 is turned OFF and the switching means S3 is turned ON, the terminal voltage V of the piezoelectric element PZT continuous to fall. Then, when the flow of current through the discharging coil L2 is stopped, i.e., when the discharging operation is completed, the terminal voltage V of the piezoelectric element PZT becomes a constant voltage. Namely, as shown in FIGS. 11(E) and 11(F), the terminal voltage V of the piezoelectric element PZT when the discharging operation is completed is changed in accordance with the ON time of the switching means S2; the longer the ON time of the switching means S2, the lower the terminal voltage V of the piezoelectric element PZT. In this case, the terminal voltage V of the piezoelectric element PZT, after the discharging operation is completed, can be changed within a range of from -200 (V) to +200 (V) by changing the ON time of the switching means S2.